The present invention relates in general to circulatory assist devices having a centrifugal blood pump with a levitated impeller, and, more specifically, to a testing method to verify whether a manufactured pump provides the proper magnetic forces on the impeller to maintain a balanced, center position of the levitated impeller.
Many types of circulatory assist devices are available for either short term or long term support for patients having cardiovascular disease. For example, a heart pump system known as a left ventricular assist device (LVAD) can provide long term patient support with an implantable pump associated with an externally-worn pump control unit and batteries. The LVAD improves circulation throughout the body by assisting the left side of the heart in pumping blood. One such system is the DuraHeart® LVAS system made by Terumo Heart, Inc., of Ann Arbor, Mich. The DuraHeart® system employs a centrifugal pump with a magnetically levitated impeller to pump blood from the left ventricle to the aorta. The impeller acts as a rotor of an electric motor in which a rotating magnetic field from a multiphase stator couples with the impeller and is rotated at a speed appropriate to obtain the desired blood flow through the pump.
The centrifugal pump employs a sealed pumping chamber. By levitating the impeller within the chamber when it rotates, turbulence in the blood is minimized. The spacing between the impeller and chamber walls minimizes pump-induced hemolysis and thrombus formation. The levitation is obtained by the combination of a magnetic bearing and a hydrodynamic bearing. For the magnetic bearing, the impeller typically employs upper and lower plates having permanent magnetic materials for interacting with a magnetic field applied via the chamber walls. For example, a stationary magnetic field may be applied from the upper side of the pump housing to attract the upper plate while a rotating magnetic field from the lower side of the pump housing (to drive the impeller rotation) attracts the lower plate. The hydrodynamic bearing results from the action of the fluid between the impeller and the chamber walls while pumping occurs. Grooves may be placed in the chamber walls to enhance the hydrodynamic bearing (as shown in U.S. Pat. No. 7,470,246, issued Dec. 30, 2008, titled “Centrifugal Blood Pump Apparatus,” which is incorporated herein by reference). The magnetic and hydrodynamic forces cooperate so that the impeller rotates at a levitated position within the pumping chamber.
A particular pump design will specify the strength and placement of all the interacting magnets in such a way that the impeller receives a desired balancing force when it is at the centered (i.e., levitated) position. Each magnet and the various plastic parts can be tested for compliance with their design specifications in an attempt to make sure that the pump will perform as intended. During assembly, however, small variations both from assembly tolerances and from aggregate small errors in the components can appear which impair the magnetic balance.
The magnetic balance is not intended to be perfect. Impeller rotation in the presence of a pumped fluid (e.g. blood) is necessary so that the hydrodynamic and magnetic forces can cooperate to achieve the balanced position. When the pump is empty and the impeller is not rotating, the lack of hydrodynamic force means that the impeller is not suspended but is magnetically held against one of the sides of the chamber. Thus, visual inspection cannot reveal whether a satisfactory magnetic balance is present. It would be desirable to verify proper force balancing without requiring pump operation with a fluid.